This invention relates to a method and apparatus for reducing the gray scale resolution of a digitized image.
In certain operations, it is necessary to produce a digitized image of a document, which image is further processed or transmitted to a utilization device. For example, in a particular banking system, it is necessary to produce digitized images of documents such as checks or deposit slips which digitized images are utilized in the system for banking and accounting purposes. One of the problems with at least some of the checks is that they contain background pictorial scenes which do not contain information which is useful within the system and these scenes have to be eliminated from the associated digitized images so as to minimize, for example, the associated transmission times and storage requirements for these images.
The pictorial scenes on the checks are generally faint or light in color or density compared to the useful data such as monetary amounts or pre-printed account numbers, for example, on the checks. When a check is scanned to produce a digitized image thereof, the scanning is done by dividing the check into a plurality of picture elements (which will be referred to hereinafter as "pixels"), with each pixel having a particular gray level associated therewith. For example, in the embodiment to be described herein, a pixel can have any one of 64 gray levels associated therewith, ranging from completely black to completely white, although the particular number of levels can be changed to suit particular applications. With 64 gray levels, 6 binary bits of data are required to describe each pixel, and with an image resolution of 154 pixels per inch, for example, it can be appreciated that a large amount of storage space would be required to store the digitized image of a check which typically is approximately 3 inches by 6 inches in area.
It was noticed that the background or pictorial scenes on a check change very slowly in gray levels when proceeding from one pixel to another pixel as compared to the gray levels associated with adjacent pixels which contain the desired useful data such as account number or amount. For example, there is a very abrupt change in the gray levels when proceeding from a background pixel to a pixel containing a portion of an account number. This feature, along with comparing each pixel with a surrounding group of pixels included in an examination area or "window", provides a basis for reducing the number of gray levels associated with a particular digitized image.